Sodium alum

Sodium aluminium sulfate is the inorganic compound with the chemical formula NaAl(SO4)2·12H2O (sometimes written Na2SO4·Al2(SO4)3·24H2O). Also known as soda alum, sodium alum, or SAS, this white solid is used in the manufacture of baking powder and as a food additive.

Sodium alum
Names
IUPAC name
Aluminium sodium bis(sulfate) — water (1:12)
Other names
Sodium alum
Soda alum
E521
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.239
EC Number
  • 233-277-3
E number E521 (acidity regulators, ...)
UNII
Properties[1]
NaAl(SO4)2·12H2O
Molar mass 458.28 g/mol
Appearance white crystalline powder
Density 1.6754 (20 °C)
Melting point 61 °C (142 °F; 334 K)
208 g/100 ml (15 °C)
1.4388
Structure[2]
Cubic, cP96
Pa3, No. 205
a = 1221.4 pm
Octahedral (Na+)
Octahedral (Al3+)
Hazards
Flash point non-flammable
Related compounds
Other cations
 Ammonium aluminium sulfate
Potassium aluminium sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Properties

Like its potassium analog, sodium aluminum sulfate crystallizes as the dodecahydrate in the classical cubic alum structure.

Sodium alum is very soluble in water, and is extremely difficult to purify. In the preparation of this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C.[3]

Production and natural occurrence

Sodium aluminum sulfate is produced by combining sodium sulfate and aluminium sulfate. An estimated 3000 ton/y are (2003) are produced worldwide.

The dodecahydrate is known in mineralogy as alum-(Na).[4][5] Two other rare mineral forms are known: mendozite (undecahydrate)[6] and tamarugite (hexahydrate).[7]

Uses

In the US, sodium aluminum sulfate is combined with sodium bicarbonate and monocalcium phosphate in typical formulations of double acting baking powder.[8]

Sodium alum is also used as an acidity regulator in food, with E number E521.

Sodium alum is also a common mordant for the preparation of hematoxylin solutions for staining cell nuclei in histopathology.

It is also used as a flocculant in water treatment and disinfection, but its relatively crude, caustic action makes it more suitable for industrial applications.[9]

References
  1. Weast, Robert C., ed. (1981). CRC Handbook of Chemistry and Physics (62nd ed.). Boca Raton, FL: CRC Press. p. B-146. ISBN 0-8493-0462-8..
  2. Cromer, D. T.; Kay, M. I.; Larson, A. C. (1967), "Refinement of the alum structures. II. X-ray and neutron diffraction of NaAl(SO4)2·12H2O, γ-alum", Acta Crystallogr., 22 (2): 182–87, doi:10.1107/S0365110X67000313.
  3. Chisholm 1911, p. 767.
  4. Burke, Ernst A.J. (2008), "Tidying up mineral names: an IMA-CNMNC scheme for suffixes, hyphens and diacritical marks" (PDF), Mineralogical Record, 39 (2): 131–35.
  5. Alum-(Na), WebMineral.com, retrieved 2009-11-28.Alum-(Na), MinDat.org, retrieved 2009-11-28.
  6. Mendozite, WebMineral.com, retrieved 2009-11-28.Mendozite, MinDat.org, retrieved 2009-11-28.
  7. Tamarugite, WebMineral.com, retrieved 2009-11-28.Tamarugite, MinDat.org, retrieved 2009-11-28.
  8. Otto Helmboldt, L. Keith Hudson, Chanakya Misra, Karl Wefers, Wolfgang Heck, Hans Stark, Max Danner, Norbert Rösch "Aluminum Compounds, Inorganic" in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim.doi:10.1002/14356007.a01_527.pub2
  9. "Products of the Sodium Hydroxide Tree" (PDF). WorldChlorine.org. Retrieved 17 June 2019.
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